Investors have long queried the strategic rationale for French billionaire Vincent Bolloré’s industry-spanning conglomerates. Now the French judiciary is starting to ask questions too.

Prosecutors are investigating whetherBolloré Group,
BOL -0.06%
the listed family company of which Mr. Bolloré is chairman and chief executive, bribed politicians in Africa by offering them reduced-price public relations through ad agency group Havas, which it controlled at the time. The suspicion is that, in return, another Bolloré division won contracts to operate container ports.

Mr. Bolloré himself is being detained, it was revealed Tuesday. The group issued a statement denying “illegal actions,” claiming the services were provided “in full transparency” and welcoming the chance to “fully cooperate with the judicial authorities.”

Mr. Bolloré is known for his use of shareholdings to control more assets than he owns. He indirectly controls 55% of a listed entity called
Financière de l’Odet,
ODET 0.53%
whose sole asset is 64% of Bolloré. And Bolloré owns a big logistics business in North Africa, which has the contracts under investigation; some loss-making ventures involving lithium batteries and electric cars; and 15% of
Vivendi,
VIVHY 1.20%
an illustrious French media group that has as its most valuable asset Universal Music, the world’s largest music company.

Since building the stake in 2015, Mr. Bolloré has used Vivendi’s balance sheet as his primary investment vehicle. Last year, he outraged corporate governance purists by using Vivendi to buy Havas, which is run by son Yannick, from Bolloré. Last week the dynastic intentions of that move became clear when Vincent Bolloré unexpectedly stepped down as chairman of Vivendi to make way for his son.

Vincent Bolloré also used Vivendi’s cash pile to accumulate a controversial 24% stake in
Telecom Italia,
Italy’s former telecom monopoly, as well as a 19% stake in
Mediaset,
the ailing TV company controlled by former Prime Minister Silvio Berlusconi. Vivendi’s control of Telecom Italia looked assured until activist investor Elliott Management took a stake and starting pushing an alternative strategy last month. A proxy battle for the board should come to a head at an extraordinary general meeting next month.

When asked why Universal Music belongs in the same conglomerate as Havas, Mediaset or Telecom Italia, Vincent Bolloré has talked airily of building a European media powerhouse. His aides privately explain that companies collaborate better when they share the same parent. The allegations around Africa put a new spin on that notion of collaboration.